Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/editor/src/display_map/flap_map.rs

@@ -36,7 +36,13 @@ impl FlapSnapshot {
         while let Some(item) = cursor.item() {
             match Ord::cmp(&item.flap.range.start.to_point(snapshot).row, &row.0) {
                 Ordering::Less => cursor.next(snapshot),
-                Ordering::Equal => return Some(&item.flap),
+                Ordering::Equal => {
+                    if item.flap.range.start.is_valid(snapshot) {
+                        return Some(&item.flap);
+                    } else {
+                        cursor.next(snapshot);
+                    }
+                }
                 Ordering::Greater => break,
             }
         }

crates/editor/src/display_map/fold_map.rs

@@ -20,6 +20,8 @@ pub struct FoldPlaceholder {
     pub render: Arc<dyn Send + Sync + Fn(FoldId, Range<Anchor>, &mut WindowContext) -> AnyElement>,
     /// If true, the element is constrained to the shaped width of an ellipsis.
     pub constrain_width: bool,
+    /// If true, merges the fold with an adjacent one.
+    pub merge_adjacent: bool,
 }
 
 impl FoldPlaceholder {
@@ -30,6 +32,7 @@ impl FoldPlaceholder {
         Self {
             render: Arc::new(|_id, _range, _cx| gpui::Empty.into_any_element()),
             constrain_width: true,
+            merge_adjacent: true,
         }
     }
 }
@@ -374,8 +377,11 @@ impl FoldMap {
 
                     assert!(fold_range.start.0 >= sum.input.len);
 
-                    while folds.peek().map_or(false, |(_, next_fold_range)| {
-                        next_fold_range.start <= fold_range.end
+                    while folds.peek().map_or(false, |(next_fold, next_fold_range)| {
+                        next_fold_range.start < fold_range.end
+                            || (next_fold_range.start == fold_range.end
+                                && fold.placeholder.merge_adjacent
+                                && next_fold.placeholder.merge_adjacent)
                     }) {
                         let (_, next_fold_range) = folds.next().unwrap();
                         if next_fold_range.end > fold_range.end {